1. Field of the Invention
The present invention relates to a method for manufacturing a silicon wafer which has an oxide precipitate density to allow a sufficient gettering capability, and a method for growing a silicon single crystal used as a raw material for the silicon wafer. Particularly, the invention relates to a method for growing a silicon single crystal to effectively manufacture a silicon single crystal having a body portion comprising an oxide precipitation promoting region (PV region) and/or oxide precipitation inhibiting region (PI region).
2. Description of Related Art
As a method for manufacturing a silicon single crystal as a raw material for a silicon wafer, a method for growing a silicon single crystal by the Czochralski method (hereafter referred as the CZ method) has been heretofore known.
It is well known that silicon single crystals manufactured by the CZ method include fine defects called grown-in defects. The occurrence of the grown-in defects becomes apparent during the manufacturing process of semiconductor devices. FIG. 1 is a schematic cross-sectional view for explaining a radial distribution of defects included in a silicon single crystal obtained by using the CZ method. As shown in FIG. 1, the grown-in defects of the silicon single crystal grown by the CZ method comprise void defects which are called infrared scattering defects or crystal originated particles (COPs), and fine dislocations which are called dislocation clusters. The void defects have a size on the order of 0.1 to 0.2 μm, and the fine dislocations have a size on the order of 10 μm.
In the silicon single crystal illustrated in FIG. 1, oxygen-induced stacking faults (hereafter referred as OSFs) occur in the ring form region having a diameter of about two-thirds of the outside diameter of the single crystal. In a region inside an OSF developing region in which the OSFs occur, infrared scattering defects on the order of 105 to 106 pieces/cm3 are detected (infrared scattering defect developing region). Outside of the OSF developing region, there is a region including dislocation clusters on the order of 103 to 104 pieces/cm3 (dislocation cluster developing region).
FIG. 2 is a drawing for explaining the state of a defect distribution at a cross section of the silicon single crystal grown by crystal pulling gradually decreasing its pulling rate. FIG. 1 is a cross-sectional view of the silicon single crystal grown at a pulling rate corresponding to the pulling rate shown by line A in FIG. 2.
As shown in FIG. 2, at a stage of fast pulling rate, the ring-shaped OSF developing region occurs on the periphery of the crystal. The inside of the OSF developing region comprises the infrared scattering defect developing region in which infrared scattering defects occur in large numbers. Along with decreasing pulling rate, the diameter of the OSF developing region decreases gradually, so that the dislocation cluster developing region in which the dislocation clusters occur appears outside the OSF developing region. Finally, the OSF developing region disappears and the dislocation cluster developing region appears in the entire cross section of the crystal.
Outside the ring-shaped OSF developing region, contacting the OSF developing region, there is an oxide precipitation promoting region (PV region) capable of forming oxide precipitates called bulk micro defects (BMDs). In the interstices between the oxide precipitation promoting region and the dislocation cluster developing region, there is an oxide precipitation inhibiting region (PI region) generating no BMDs. The oxide precipitation promoting region (PV region), the oxide precipitation inhibiting region (PI region), and the ring-shaped OSF developing region are each a defect-free region in which the grown-in defects are very few in number.
Compared with silicon single crystals, in which dislocation clusters are detected, silicon single crystals in which infrared scattering defects are detected have little adverse effect on semiconductor devices and are excellent in productivity because they can be grown by relatively fast pulling rate. However, as integrated circuits have become increasingly miniaturized in recent years, a decrease in dielectric strength of oxide has been pointed out due to infrared scattering defects, so that there is an increasing demand for a high-quality silicon single crystal which comprises a defect-free region where neither infrared scattering defects nor dislocation clusters are detected.
As a method for growing a silicon single crystal comprising a defect-free region, a method has been proposed for growing a silicon single crystal by using a hot zone structure where, for instance, a thermal gradient (Gc) at the central portion of the crystal is equal to or larger than a thermal gradient (Ge) at the periphery of the crystal (Gc≧Ge) (see, for example, Patent Document 1: PCT International Publication No. WO 2004/083496). FIG. 3 is a drawing for explaining the state of a defect distribution at the cross section of a silicon single crystal grown by pulling with gradually decreasing pulling rate. This single crystal is grown by using a crystal growing apparatus having a hot zone structure where a thermal gradient (Gc) at the central portion of the crystal is equal to or larger than a thermal gradient (Ge) at the periphery of the crystal (Gc≧Ge).
As shown in FIG. 3, when the crystal is grown at a pulling rate in a range of B to C shown in FIG. 3 by using the crystal growing apparatus having a hot zone structure satisfying Gc≧Ge, by the control of a thermal gradient G in the crystal near a solid-liquid interface, it is possible to grow a silicon single crystal which provides wafers having surfaces entirely comprising a uniform defect-free region. A pulling rate range for pulling a defect-free crystal (in FIG. 3, the range of B to C) is called the pulling rate margin of the defect-free crystal.
Furthermore, Patent Document 1 discloses a technique of increasing the pulling rate margin of a defect-free crystal by using a crystal growing apparatus having a hot zone structure which satisfies Gc≧Ge and by introducing hydrogen into an atmosphere in a pulling furnace. FIG. 4 is a drawing for explaining a state of a defect distribution at a cross section of a silicon single crystal grown by using the same crystal growing apparatus as that described in FIG. 3 having the hot zone structure which satisfies the relationship Gc≧Ge. In this case, while supplying an inert gas containing hydrogen into the furnace, the single crystal is grown by crystal pulling with gradually decreasing pulling rate.
When a mixed gas of an inert gas and hydrogen is used as an atmospheric gas used for growing a single crystal, since the hydrogen inhibits the occurrence of dislocation clusters caused by interstitial atoms, a pulling rate for a defect-free region is lowered. Therefore, compared with the example shown in FIG. 3 in which hydrogen is not introduced into the pulling furnace, as shown in FIG. 4, a minimum pulling rate for pulling up a defect-free crystal decreases and a pulling rate range for pulling up a defect-free crystal (the pulling rate margin of a defect-free crystal, that is, the range of D to E shown in FIG. 4) increases.
However, even when the pulling rate margin of the defect-free crystal is increased by introducing hydrogen into the pulling furnace as described in Patent Document 1, the extent of the pulling rate margin of the defect-free crystal is still insufficient to grow a silicon single crystal comprising one region selected from among the OSF developing region, the PV region, and the PI region. Therefore, these three regions tend to be mixed in the same crystal.
As described above, the OSF developing region is the defect-free region where neither infrared scattering defects nor dislocation clusters are detected; however, when the concentration of oxygen is high, an adverse effect may be exerted on device characteristics because occurrence of OSFs as secondary defects of oxide precipitates becomes apparent. Therefore, when a silicon single crystal comprising a defect-free region has been grown, it has been needed to control the oxygen content not more than 12×1017 atoms/cm3 (ASTM-F121 1979) such that the OSFs do not occur apparently. However, when the oxygen concentration of a silicon single crystal is controlled to have low value not more than 12×1017 atoms/cm3 (ASTM-F121 1979), such a single crystal may not provide a silicon wafer having sufficient density of oxide precipitates to allow a sufficient gettering capability.
When both the PV region and PI region are mixed in a silicon single crystal, the oxide precipitation characteristics such as density, size, and DZ width of the oxide precipitates may be inhomogeneous in the surface of a silicon wafer sliced from a grown silicon single crystal. That is, when the PV region and the PI region are mixed in the wafer, oxide precipitates distribute heterogeneously in the device processing, so that portions of high gettering capability and portions of low gettering capability are mixed within a wafer. In addition, although an active region near a device surface must be free of not only infrared scattering defects and dislocation clusters, but also oxide precipitates and their secondary defects such as OSFs and punch out dislocations, the width of a region free of such defects, that is, DZ width cannot be uniform in a wafer including both PV region and PI region. The non-uniform distributions of the gettering capability (IG capability) and the DZ width result in variations in device characteristics and a decrease in the yield of device production.
To solve the problems described above caused by the mixing state of the OSF developing region, the PV region, and the PI region in the defect-free crystal, it is possible to consider to grow a silicon single crystal by using the pulling rate margin only for the individual regions of the defect-free crystal (the range of D to E shown in FIG. 4). However, since such a growing condition further reduce the narrow range of the pulling rate margin of a defect-free crystal including the OSF developing region, the PV region, and the PI region, stable manufacture of a defect free crystal comprising only one region selected from the OSF developing region, the PV region, and the PI region has been difficult in terms of industrial production.
The present invention has been made in view of such consideration and has an object to provide a method for manufacturing a silicon wafer comprising an oxide precipitation promoting region (PV region) and/or an oxide precipitation inhibiting region (PI region), which have an oxide precipitate density to allow a sufficient gettering capability.
Another object of the invention is to provide a method for growing a silicon single crystal which allows the silicon single crystal whose body portion comprises an oxide precipitation promoting region (PV region) and/or an oxide precipitation inhibiting region (PI region) to be manufactured with a high yield.